KITTERY, Maine — A union leader at Portsmouth Naval Shipyard says he’s confident the fire-damaged USS Miami can be repaired, but it’ll be several weeks before the Navy reaches conclusions on the extent of damage.

The Navy will provide an update on the nuclear-powered submarine after three separate investigations are completed in two to three weeks, Patricia Dolan, spokeswoman for the Naval Sea Systems Command in Washington, D.C., said Tuesday.

Based in Connecticut, the $900 million Los Angeles-class submarine was in dry dock at Portsmouth Naval Shipyard for an overhaul when a fire broke out last week, damaging forward compartments including the torpedo room, command and control, and crew quarters, officials said.

An investigation by the military’s legal arm, the Judge Advocate General Corps, will get to the bottom of what caused the fire, while a separate investigative team is looking at safety procedures and the Naval Criminal Investigative Service is looking into whether a crime was committed.

Paul O’Connor, president of the Metal Trades Council, said that based on what he knows, he’s hopeful the Miami will be repaired at Portsmouth and returned to the fleet.

“Our plan is get that boat back to the Navy, and back on deployment,” Connor said Tuesday.

Insulation and wiring fueled the stubborn fire and the metal hull trapped heat, turning the forward part of the sub into a superheated oven. More than a hundred firefighters worked in shifts to douse the blaze.

Big repairs wouldn’t be unprecedented on a Los Angeles-class sub.

The USS San Francisco suffered heavy damage when it hit an uncharted underwater mountain at full speed in January 2005, killing one sailor and injuring 97. The Navy ultimately chose to replace its bow with part of a decommissioned submarine, a repair that took more than three years and cost $134 million.

In this case, much of the USS Miami’s sensitive equipment had been removed and the fire didn’t damage any nuclear components, both factors that work in favor of repairing the vessel, officials say.

But the Navy said it’s premature to say whether the vessel can be repaired until the damage is fully assessed and the investigations are complete.

Working against the submarine is that it’s 22 years old, there’s no pressing threat and the Budget Control Act could cut defense spending by $100 billion in fiscal 2013, said Loren Thompson, defense analyst at the Lexington Institute, a think tank based outside Washington, D.C. Also, he said, fires as hot as the one aboard the USS Miami can cause complex weakening and destabilization of the metal structures.

The submarine was three months into a 20-month overhaul when the fire struck, so scrapping the sub would have major ramifications on the Portsmouth work force.

O’Connor said the shipyard workers don’t want to see the Navy lose a sub, especially one that was in the shipyard’s care when the fire broke out.

“Shipyarders are a tough lot,” he said. “We’re not going to roll over and give up. That’s not who we are or what we do. We’re fighters. It’s in our nature.”